I disagree about Onomah's chances. But I think if I'm correct you're saying that the academy players need to be more patient and allow themselves to bed in more? Poch has been here 4 years and we've seen one academy player get integrated, if these players are all having to wait until they're 22, while also getting zero experience from loans, before they get a sniff of the first team, can you really wonder why these players might want to leave while they're 17/18 to go to a league known for giving young players chances, and not staying at a club that claims it.
Most of it was written trying to see Pochettino's side but yes, I do think young players need to be more realistic, and patient, if they want to make it here. You're absolutely right that moves to Germany provide a good alternative, and it's certainly an attractive one for young players.
If Onomah wanted to play the role he played at the U20 World Cup then he has to convince a manager to play him there. From what I saw of his time at Villa, Bruce tried him there and then pushed him up to no 10, or played him in a midfield 3. Frankly I think he was right, his best work was in the final third. He was excellent in a deeper role with the u20s but that's the u20s and not the level he wants to play at. He might have modeled his game on Dembele but even Dembele moved positions over the years, as we all know. Dembele didn't learn how drive the ball forward and beat players effortlessly in midfield where it was more risky, he learnt that as an attacking player, with a whole midfield and defence behind him. Losing the ball to defenders, not midfielders.
Alli's another good player to look at for comparison. Pochettino quickly moved him further forward and he was incredibly effective. Now he's developing other sides of his game and is well on his way to becoming a complete player. Although the obvious answer to that is that he's been given an awful lot more minutes.
I respect Onomah's decision to go out on loan to prove himself but with the hindsight of Dembele and Winks' injuries, he should have been the one to get moved from the right to centre mid as Sissoko frequently was. It's often unexpected breaks of luck(from the player's perspective) that leads to a player getting the chance that makes them. For Kane it was Soldado and Ade being god awful, as well as a deflected freekick against Villa. Winks, until the injuries, had Dembele's injuries to thank for dramatically increasing his game time. Bentaleb had a god awful manager that luckily believed in him. Mason had just enough preseason opportunities with a new manager, iirc thanks to players on extended World Cup breaks, and a shot against Forest that even great players usually stick in the stands. Further afield would TAA wouldn't be on the plane if it weren't for Clyne's injuries and Rashford made his name after a series of injuries to other forwards, including one on the day of a match.
It's not really luck so much as biding your time and waiting for the opportunities.